CONCRETE REINFORCEMENT, REINFORCED CONCRETE AND METHODS OF REINFORCING CONCRETE

Abstract: Metal reinforcement for concrete comprises an elongate member having a surface area per unit length of at least 20 % greater than that of a round bar. The reinforcement also has surface deformations for interlock with the concrete. The deformations are of such number, shape and size that when the reinforcement is loaded axially with respect to the concrete an anchorage bond due to the deformations transmits load between concrete and reinforcement under the same conditions that load is transmitted therebetween by adhesion bond which relies on surface area as opposed to the deformations.

Description: CONCRETE REINFORCEMENT, REINFORCED CONCRETE AND METHODS OF REINFORCING CONCRETE The invention relates to reinforced concrete structures, to reinforcement for use in such structures and to methods of reinforcing concrete. Since concrete has high resistance to co pressive forces but a low resistance to tensile forces, reinforcement is normally provided in the form of one or more bars securely located in the mass of concrete in order to impart resistance to tensile forces that are usually generated within the concrete structure. Such resistance to tensile forces requires the transfer of forces in an axial direction of the reinforcement between the concrete and the reinforcement. The ability to transfer load in this way is referred to as 'bond 1 . Bond is a complex phenomenon but for practical purposes it may be considered as two distinct elements. One element is referred to as adhesion bond and is made up of glue-like adhesion as such in conjunction with surface friction between reinforcement and concrete. The second element is brought about by mechanical interlocking between deformations such as ribs or indentations in the surface of the reinforcement and is referred to as anchorage bond. Throughout this specification and the claims, the terms 'adhesion bond' and 'anchorage bond 1 have the meanings ascribed to them in this paragraph. A factor affecting the bond phenomenon is that concrete is almost inextensible while typical reinforcement requires a significant longitudinal extension before it develops any tensile load; the modulus (E) for steel is about 15 times that of concrete. As a result, it is inevitable that cracks develop in the concrete. The nature of the crack pattern can be an important factor in the effectiveness of the bond between reinforcement and concrete. In conventional reinforced concrete structures, where smooth surfaced reinforcement is employed the bond is purely an adhesion bond and performance has to be assessed accordingly. Where higher stresses are envisaged, reinforcement with a deformed surface is employed in order to achieve the higher load transfer normally associated with anchorage bond. Adhesion bond strength can be significant at relatively low loads. However once it begins to diminish because the adhesion limit has been exceeded at some locations, the adhesion bond strength falls off rapidly with increasing load and its full value is never available again even if the structure is unloaded and reloaded. In contrast, anchorage bond builds up gradually and does not normally reach significant values until the adhesion bond has broken down. In practice, little or no reliance can be placed on the adhesion bond element of a complex bond for structures incorporating deformed highly stressed reinforcement. Thus the ribs or other deformations are made sufficiently large to establish the required load transfer capacity purely by anchorage bond. The use of large ribs in turn tends to lead to relatively small numbers of large cracks in the concrete whereas larger numbers of small cracks would be desirable, particularly in relation to penetration of the concrete by contaminants and corrosion of the reinforcement and the prevention of leakage from structures designed to retain aqueous liquids.. The corrosion and crack problems in turn can result in a requirement for deeper embedment of the reinforcement within the concrete which in turn leads to a less efficient structure. Objectives of the invention are to eliminate or reduce the above mentioned disadvantages of the conventional structures. According to one aspect of the invention metal reinforcement for concrete comprises an elongate member having a surface area per unit length of at least 20% greater than that of a round bar and having surface deformations for interlock with the concrete, the deformations being of such number, shape and size that when the reinforcement is loaded axially with respect to the concrete an anchorage bond transmits load between concrete and reinforcement under the same conditions that load is transmitted therebetween by adhesion bond. In general adhesion bond and anchorage bond can be brought into balance in this way by increasing surface area and employing large numbers of small deformations as compared with conventional reinforcement. Preferably the reinforcement has a surface area per unit length of between 1.2 and 2 times that of a round bar. Typically, the reinforcement is in the form of a flat strip as a convenient way of providing a large surface area. The deformations may be raised transverse ribs or may be projections on one surface of strip material coincident with indentations on the opposite surface. Alternatively, the deformations are formed by holes through the reinforcement. According to a second aspect of the invention a method of producing reinforced concrete comprises incorporating elongate metal reinforcement into the concrete, wherein the reinforcement is selected in accordance with the criteria set forth above. According to a further aspect of the invention, metal reinforcement for concrete comprises an elongate member having a surface area per unit length such that when embedded in concrete the strength of the bond between the reinforcement and the concrete matches the design load of the reinforcement. This adjustment of the surface area can be employed to increase the bond to a level where the strength of the reinforcement is used efficiently. The reinforcement may have a smooth surface such that the whole of the bond is adhesion bond. The invention will now be described in greater detail with reference to the accompanying drawings in which: Figures 1 to 5 are illustrations of various bar cross sectional shapes, of which Figures 1 and 2 are conventional prior art; Figures 6 to 13 illustrate various forms of deformation which may be applied to concrete; Figures 14 and 15 illustrate the use of prior art reinforcement and reinforcement according to the invention in a concrete beam; Figures 16 and 17 correspond to Figures 14 and 15 but show a different application. Figures l and 2 illustrate round and square bar cross sections which may be employed for concrete reinforcement. The round section is most conventional but the square section has also been proposed. With the round bar, the surface area per unit length is at a minimum for a given cross sectional area and thus strength of bar. The square bar by coincidence gives a slightly increased surface area ratio. Figures 3, 4 and 5 illustrate cross sections for reinforcing bars which have aspect ratios of 2:1, 3:1 and 4:1 respectively. With these bars, the surface area ratio as compared with a round bar is increased in each case by approximately 20%, 30% and 41%. The 2:1 aspect ratio is seen as a reasonable minimum for a worthwhile increase in the surface area ratio. The increase is actually 19.7% but for practical purposes may be treated as 20%. Still higher aspect ratios may be employed up to a practical limit for thickness of the bar. For example, an aspect ratio of 10:1 gives an area ratio of almost 2:1 (actually 1.96:1). The increase in area naturally leads to a corresponding increase in the adhesion bond between the reinforcement and concrete in which it is embedded. Sizes of reinforcement are normally given by simple numbers representing the nominal diameter. In this specification we adopt the same terminology so far as possible. For example, a bar measuring 3.5mm x 14mm has approximately the same area as a 16mm diameter round bar and is referred to as size 16. Bars or rods of smaller sizes, even if of round cross section, have a larger surface area for a given cross section than bars of larger sizes simplifying the task of increasing the adhesion bond to a satisfactory level for the purposes of the invention. Similarly, bars of smaller sizes tend to result in larger numbers of smaller cracks, an effect which is augmented by use of this invention. Thus the invention is particularly applicable to bars of smaller sizes, for example sizes 6 to 16. Also the smaller aspect ratios are more suitable for use with smaller sizes. Figures 6 and 7 show a short length of flat bar with one form of surface deformation. This deformation is broadly in the nature of intersecting diagonal grooves in both the major surfaces of the reinforcement. Such surfaces can be established for example by hot rolling using rolls having suitable surfaces. The result is broadly a knurled surface incorporating a large number of closely spaced deformations. These deformations provide an anchorage bond with concrete. The small deformations produce an anchorage bond which typically transfers less load than can be transferred with larger deformations but which is effective at smaller longitudinal displacements between concrete and reinforcement. The relatively small displacement is associated with a cracking pattern for the concrete involving larger numbers of smaller cracks than occur with conventional ribbed reinforcement. Breakdown of adhesion bond is displacement related so the adhesion bond can remain effective under the same conditions of load and displacement under which the anchorage bond becomes effective with the deformations of Figures 6 and 7. With such an arrangement, the anchorage bond can be relied upon and allowed for in calculations. Particularly when the adhesion bond is increased by using a large aspect ratio for the strip, the adhesion bond can assume sufficient significance to compensate for the expected reduction in anchorage bond. Also, the reduction in anchorage bond associated with small deformations is itself mitigated by the increased area over which the anchorage bond applies. In general, the reduced displacements necessary to achieve substantial load transfer between concrete and reinforcement results in a more efficient reinforced concrete structure. Figures 8 and 9, Figures 10 and 11 and Figures 12 and 13 show alternative arrangements for deformations. In particular. Figures 8 and 9 show small diagonal ribs. Figures 10 and 11 show depressions on one major surface of the reinforcement coincident with projections on the other surface. Deformations of this kind can be formed by hot rolling. The arrangement of Figures 12 and 13 shows holes through the reinforcement from one major surface to the other to constitute the deformed surface thereof. Flat strip is the most convenient form for the reinforcement to increase its surface area per unit length but a range of alternative shapes may be employed instead. In some conditions, the deformation of the surface of the reinforcement may not be necessary. In particular, it may be possible to increase the surface area sufficiently to provide sufficient adhesion bond for transfer of the load between concrete and reinforcement. Although the primary objective of the invention is to improve total bond at small displacements, several ancillary but important advantages arise by use of the invention. Two such advantages are illustrated in Figures 14 to 17. Figures 14 and 15 illustrate reinforced concrete beams with reinforcement confined for the purposes of illustration to the lower edges of the beam which will be in tension in use. Figure 14 shows conventional reinforcement while Figure 15 shows reinforcement conforming to the invention. For the purposes of efficiency of the structure, the reinforcement should be as low down as possible in the beam. However, for preventing expansion caused by corrosion at the surface of the reinforcement from breaking the lower surface of the beam away from the rest of the beam, a predetermined depth of cover is required for the concrete. By choosing the same depth of cover below the reinforcement in both cases, the centre line of the reinforcement according to the invention is lower than that of the prior art, resulting in an improved efficiency for the structure. Also, reducing the distance Acr from the edge of the reinforcement to the centre point at the surface of the beam helps to reduce crack widths in Figure 15 compared with Figure 14. In addition, the improved crack patterns associated with the reinforcement according to the invention may permit the reinforcement to be placed even nearer to the surface of the beam. Figures 16 and 17 illustrate similar situations with two reinforcement elements in an edge of a concrete structure. Two spaced elements can be more effective than a single larger element but in a narrow concrete structure the minimum external cover requirement may place the two elements too close together resulting in a line of weakness in the concrete running between the two elements. However, by using narrow strips according to the invention, required spacings can be maintained thus permitting the use of two separate reinforcing elements. A further advantage is that reinforcing bars in the form of strips can be bent around sharper angles in a selected direction without fear of cracking or similar damage to the reinforcement. Energy requirements for bending are also reduced. A still further advantage is that when intersecting reinforcing elements are to be welded together, greater areas of contact are available for the weld. This applies when reinforcing elements intersect as in a mesh and when reinforcing elements are overlapped in an end to end join. Small cross section reinforcement is conveniently stored and handled in coil form but larger sections have to be in straight lengths due to difficulties of bending and straightening. With a wide but thin cross section in accordance with the invention, larger sections can be coiled and uncoiled conveniently, thus avoiding the disadvantages associated with employment of relatively short finite lengths.

Claims: CLAIMS 1. Metal reinforcement for concrete comprising an elongate member having a surface area per unit length of at least 20% greater than that of a round bar and having surface deformations for interlock with the concrete, the deformations being of such number, shape and size that when the reinforcement is loaded axially with respect to the concrete an anchorage bond (as herein defined) transmits load between concrete and reinforcement under the same conditions that load is transmitted therebetween by adhesion bond. 2. Reinforcement as claimed in claim 1 having a surface area per unit length of between 1.2 and 2 times that of a round bar. 3. Reinforcement as claimed in claim 1 or claim 2 wherein the metal is steel. 4. Reinforcement as claimed in any preceding claim wherein the reinforcement is in the form of a flat strip. 5. Reinforcement as claimed in any preceding claim wherein the deformations are raised transverse ribs. 6. Reinforcement as claimed in any one of claims 1 to 4 wherein the deformations are in the form of projections on one surface of strip material coincident with indentations on the opposite surface. 7. Reinforcement as claimed in any one of claims 1 to 4 wherein the deformations are formed by holes through the reinforcement. 8. Reinforced concrete incorporating metal reinforcement as claimed in any preceding claim. 9. A method of producing reinforced concrete by incorporating elongate metal reinforcement into the concrete wherein the reinforcement is selected in accordance with the criteria set forth in any one of claims 1 to 8. 10. Metal reinforcement for concrete comprising an elongate member having a surface area per unit length such that when embedded in concrete the strength of the bond between the reinforcement and the concrete matches the design load of the reinforcement. 11. Reinforcement as claimed in claim 10 wherein the metal is steel. 12. Reinforcement as claimed in claim 10 or claim 11 wherein the reinforcement has a smooth surface such that the whole of the bond is adhesion bond. 13. Reinforcement as claimed in any one of claims 10 to 12 wherein the reinforcement is formed of flat strip. 14. Reinforced concrete incorporating reinforcement according to any one of claims 10 to 13. 15. A method of reinforcing concrete comprising incorporating in the concrete metal reinforcement as set forth in any one of claim 10 to 13.

Inventors: CLAPSON JOHN DAVID, CLAPSON, JOHN, DAVID
Applicants: ASW LTD, CLAPSON JOHN DAVID, ASW LIMITED, CLAPSON, JOHN, DAVID
Application year: 1994
Main classification: E04C5
IPC codes: E04C5